1. Field of the Invention
The present invention relates to an image forming apparatus of a copying machine or printer which uses an electrophotographic system or an electrostatic recording system. More particularly, the present invention is related to an image forming apparatus capable of recognizing the density of a toner image transferred to an intermediate transfer belt corresponding to a position in the circumferential direction of the intermediate transfer belt.
2. Description of the Related Art
Conventionally, for example, as an image forming apparatus capable of forming a full color image, the following direct transfer-type and intermediate transfer-type image forming apparatuses are known. In a direct transfer-type image forming apparatus, a toner image formed on a single or a plurality of photosensitive drums is transferred to a transfer material carried on a belt member (hereinafter referred to as a “transfer belt”), which is a transfer material bearing member capable of rotating in the circumferential direction. In an intermediate transfer-type image forming apparatus, a toner image formed on a single or a plurality of photosensitive drums is transferred once (primary transfer) to a belt member (hereinafter referred to as an “intermediate transfer belt”), which is an intermediate transfer member capable of rotating in the circumferential direction. Subsequently, in an intermediate transfer-type image forming apparatus, the toner image on the intermediate transfer belt is transferred (secondary transfer) to a transfer material. In an intermediate transfer-type image forming apparatus, it is easy to form an image on various recording materials, and thus the selectivity of the recording material can be increased.
In an image forming apparatus, the density of a patch image is detected by an optical sensor and the image density of the colors is controlled by a control of toner replenishment of developer by a patch detection system or the like, and registration deviation is detected by the optical sensor and registration correction is performed.
Given the above, a constitution of an image forming apparatus which has an intermediate transfer belt and controls the image density based on detection of a patch density by an optical sensor can be envisioned. A technology called ground correction is necessary for the measurement of the patch density transferred to a transfer surface of an intermediate transfer belt by an optical sensor. During measurement of the patch density, ground correction has an effect of canceling the influence from the gloss of the surface of the ground on the patch density, while taking the condition of the ground of the intermediate transfer belt to which the toner is transferred into account. The condition of the ground of the intermediate transfer belt includes, for example, variation over time in the gloss of the transfer surface of the intermediate transfer belt and gloss irregularities in the intermediate transfer belt within one round. In particular, as an invention for canceling the influence from gloss irregularities in the intermediate transfer belt within one round on the patch density, the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 05-150574 has been proposed.
In the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 05-150574, before measuring the patch density, a control apparatus reads one round of the transfer surface of the intermediate transfer belt which becomes the ground of the toner image via a density sensor, and then stores the phase of the intermediate transfer belt and the output value of the density sensor for each phase. Subsequently, the control apparatus references the phase of the transfer surface of the intermediate transfer belt and the output value of the density sensor in each phase which were stored to ascertain the output value of the density sensor corresponding to the transfer surface of the intermediate transfer belt of a certain phase which becomes the ground of the toner patch. The control apparatus then performs ground correction to obtain the density of the toner patch.
Further, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 05-150574, in order for the control apparatus to become able to recognize the phase of the transfer surface of the intermediate transfer belt, marks are attached to the intermediate transfer belt at prescribed intervals in the circumferential direction, and a mark detecting sensor which detects the marks is arranged facing the intermediate transfer belt. A calculation unit within the control apparatus determines the current phase of the intermediate transfer belt based on the elapsed time from when the prescribed marks on the intermediate transfer belt are detected by the mark position detecting sensor.
However, when obtaining a profile of one round of the intermediate transfer belt using an optical sensor, a reference position which becomes the reference of the circumferential direction is necessary for the intermediate transfer belt. If there is one mark attached to such a reference position of the intermediate transfer belt, a wait time until the mark comes to the location of the mark detecting sensor occurs. Therefore, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 05-150574, in order to reduce the wait time until the mark comes to the location of the mark detecting sensor, a plurality of marks of the same shape and same pattern are attached to the intermediate transfer belt.
However, the belt position detecting marks cannot be distinguished from each other, and after a power source is suddenly turned OFF or after a process to eliminate a paper jam, the reference position is lost, and the reference position and the profile of one round of the intermediate transfer belt must be retaken, and thus a wait time may occur.
The present invention provides an image forming apparatus which can reduce the time for determining the position of a belt member.